Method for forming semiconductor microstructure, semiconductor device fabricated using this method, method for fabricating resonance tunneling device, and resonance tunnel device fabricated by this method

ABSTRACT

The method for forming a semiconductor microstructure of this invention includes the steps of: forming a mask pattern having a first opening and a second opening on a substrate having a semiconductor layer as an upper portion thereof; and selectively etching the semiconductor layer using the mask pattern to form a semiconductor microstructure extending in a first direction parallel to a surface of the substrate, wherein, in the step of selectively etching the semiconductor layer, an etching rate in a second direction vertical to the first direction and parallel to the surface of the substrate is substantially zero with respect to an etching rate in the first direction, and a width of the semiconductor microstructure is substantially equal to a shortest distance between the first opening and the second opening in the second direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for forming a semiconductormicrostructure and a method for fabricating a resonance tunnelingdevice. More particularly, the present invention relates to a method forforming a silicon (Si) microstructure on an SIMOX (separation byimplanted oxygen) substrate by use of crystal anisotropic etching and amethod for fabricating a resonance tunneling device having such asilicon microstructure.

2. Description of the Related Art

In recent years, quantization functional devices utilizing a quantumeffect have been studied and developed. Among these quantizationfunctional devices, a resonance tunnel diode utilizing a resonancetunnel effect has been proposed and studied. A method for fabricatingsuch a resonance tunnel diode is disclosed in Japanese Laid-Open PatentPublication No. 7-312419, where an etching mask having two etchingwindows which are close to each other and parallel to the <211>direction is formed on a (110) SIMOX substrate, and a narrow Simicrostructure is formed under the etching mask by controlling an Sietching amount by crystal anisotropic etching, to be used as a quantumwell of a resonance tunnel diode.

The above conventional method for forming a semiconductor microstructurehas the following problems. First, due to limitations of the presentalignment technique or an error in the measurement of the orientation ofa substrate, the direction where the etching windows extend and the<211> direction of the substrate may not be strictly parallel, but maybe deviated from each other to some extent. When the etching mask haslong etching windows for the formation of a long Si microstructure, theetching proceeds toward a direction parallel to the <211> direction,finally reaching an edge of the etching mask, and thus an Simicrostructure cut halfway may be formed.

Secondly, in the conventional lithographic technique, the space betweenthe etching windows cannot be made so narrow for the reason of thelimitation of the light wavelength. A very narrow space between theetching windows is also difficult for other reasons. For example, if thespace is very narrow, extremely high a precision is required at thealignment in the subsequent lithographic step. A method employingelectron beam drawing and the like has been proposed to overcome theseproblems. However, this method does not seem to be practical inconsideration of the drawing cost and throughput. This method entailsdifficulty even when used for research.

Thirdly, in the conventional method for forming a semiconductormicrostructure, though the width of the semiconductor microstructure iscontrolled by the etching amount, the etching amount is difficult to bemonitored during etching.

Therefore, there is a strong need in the art for a method forfabricating a semiconductor microstructure and a resonance tunnelingdevice which overcomes the above-mentioned problems associated withconventional methods.

SUMMARY OF THE INVENTION

The present invention provides a method for forming a semiconductormicrostructure used as a quantum well of a resonance tunnel diode by aconventional lithographic technique at low cost and high throughput.

The present invention also provides a method for fabricating a resonancetunnel diode having such a semiconductor microstructure.

In particular, the present invention provides a method for forming asemiconductor microstructure with good controllability of the widththereof where the semiconductor microstructure is not cut halfway evenif the lithographic alignment is deviated from the crystal orientationof a substrate to some extent.

The method for forming a semiconductor microstructure of this inventionincludes the steps of: forming a mask pattern having a first opening anda second opening on a substrate having a semiconductor layer as an upperportion thereof; and selectively etching the semiconductor layer usingthe mask pattern to form a semiconductor microstructure extending in afirst direction parallel to a surface of the substrate, wherein, in thestep of selectively etching the semiconductor layer, an etching rate ina second direction vertical to the first direction and parallel to thesurface of the substrate is substantially zero with respect to anetching rate in the first direction, and a width of the semiconductormicrostructure is substantially equal to a shortest distance between thefirst opening and the second opening in the second direction.

In one embodiment of the invention, the orientation of the substrate is(110), the first direction is <211>, and the second direction is <111>.

In another embodiment of the invention, the substrate is an SOIsubstrate.

In still another embodiment of the invention, the substrate is a SIMOXsubstrate.

In still another embodiment of the invention, the mask pattern has astripe-shaped portion between the first opening and the second opening,and a length direction of the stripe-shaped portion does not correspondto the first direction.

In still another embodiment of the invention, the method furtherincludes the step of forming a first oxide film and a second oxide filmformed in parallel to each other in the first direction to sandwich thesemiconductor layer, wherein, in the step of selectively etching thesemiconductor layer, etching in the first direction is terminated by thefirst oxide film and the second oxide film.

In still another embodiment of the invention, the semiconductor layer ismade of Si.

In still another embodiment of the invention, an oxide film is formedunder the semiconductor layer of the substrate.

In still another embodiment of the invention, the step of selectivelyetching the semiconductor layer includes crystal anisotropic etching.

According to another aspect of the invention, a method for fabricating aresonance tunneling device including a silicon thin plate having sidewalls and a width thin enough to serve as a quantum well, a pair oftunnel barriers formed on the side walls of the silicon thin plate, anda pair of electrodes formed to sandwich the pair of tunnel barriers isprovided. The silicon thin plate is formed in the steps of: forming anoxide film having a first opening and a second opening on a substratehaving a silicon layer as an upper portion thereof; and selectivelyetching the silicon layer using the oxide film as a mask to form thesilicon thin plate extending in a first direction parallel to a surfaceof the substrate, wherein, in the step of selectively etching thesilicon layer, an etching rate in a second direction vertical to thefirst direction and parallel to the surface of the substrate issubstantially zero with respect to an etching rate in the firstdirection, and a width of the silicon thin plate is substantially equalto a shortest distance between the first opening and the second openingin the second direction.

In one embodiment of the invention, the pair of electrodes are formed bydry etching using the oxide film as a mask in a self-alignment fashion.

According to still another aspect of the invention, a semiconductordevice is provided. The device includes: a double barrier structureincluding a silicon thin plate having side walls and extending in afirst direction, and a pair of first oxide films formed on the sidewalls of the silicon thin plate; and a second oxide film formed to coverthe silicon thin plate, wherein the second oxide film has a firstopening and a second opening, and a shortest distance between the firstopening and the second opening in a second direction vertical to thefirst direction and parallel to a surface of the second oxide film issubstantially equal to a width of the silicon thin plate in the seconddirection.

According to still another aspect of the invention, a resonancetunneling device is provided. The device includes: a silicon thin platehaving side walls and a width thin enough to serve as a quantum well; apair of tunnel barriers formed on the side walls of the silicon thinplate; a pair of electrodes formed to sandwich the pair of tunnelbarriers; and an oxide film formed to cover the silicon thin plate,wherein the oxide film has a first opening and a second opening, and ashortest distance between the first opening and the second opening in asecond direction vertical to the first direction and parallel to asurface of the second oxide film is substantially equal to a width ofthe silicon thin plate in the second direction.

Thus, the invention described herein makes possible the advantages of(1) providing a method for forming a semiconductor microstructure usedas a quantum well of a resonance tunnel diode at low cost and highthroughput, and (2) providing a method for fabricating a resonancetunnel diode having such a semiconductor microstructure.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D show a semiconductor microstructure formed by a method ofExample 1 in accordance with the present invention: FIG. 1A is a planview, FIG. 1B is a perspective view, and FIG. 1C and 1D are sectionalviews taken along line 1C--1C and line 1D--1D of FIG. 1A, respectively.

FIG. 2A is a plan view illustrating a stage of crystal anisotropicetching of the method of Example 1 according to the present invention.FIGS. 2B and 2C are sectional views taken along line 2B--2B and line2C--2C of FIG. 2A, respectively.

FIG. 3A is a plan view illustrating another stage of crystal anisotropicetching of the method of Example 1 according to the present invention.FIGS. 3B and 3C are sectional views taken along line 3B--3B and line3C--3C of FIG. 3A, respectively.

FIG. 4A is a plan view illustrating yet another stage of crystalanisotropic etching of the method of Example 1 according to the presentinvention. FIGS. 4B and 4C are sectional views taken along line 4B--4Band line 4C--4C of FIG. 4A, respectively.

FIG. 5A is a plan view illustrating the final stage of crystalanisotropic etching of the method of Example 1 according to the presentinvention. FIGS. 5B and 5C are sectional views taken along line 5B--5Band line 5C--5C of FIG. 5A, respectively.

FIGS. 6A to 6D show etching masks used in the method according to thepresent invention.

FIG. 7 shows the size of a typical etching mask used in the methodaccording to the present invention.

FIG. 8 is a plan view of another semiconductor microstructure formed bythe method according to the present invention.

FIGS. 9A to 9D are plan views illustrating stages of crystal anisotropicetching of a method of Example 2 according to the present invention.

FIGS. 10A to 10D are plan views and FIG. 10E is a sectional view,illustrating stages of modified crystal anisotropic etching of themethod of Example 2 according to the present invention.

FIGS. 11A to 11C show a semiconductor device (resonance tunnelingdevice) formed by a method of Example 3 according to the presentinvention: FIG. 11A is a perspective view, FIG. 11B is a sectional view,and FIG. 11C is a plan view.

FIG. 12 is a graph showing the electric characteristics of thesemiconductor device shown in FIGS. 11A to 11C.

FIGS. 13A to 13F are perspective views illustrating steps forfabricating the semiconductor device shown in FIGS. 11A to 11C.

FIG. 14 is an SEM photograph of a semiconductor microstructure after thecrystal anisotropic etching according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (EXAMPLE 1)

A method for forming a semiconductor microstructure of Example 1according to the present invention will be described. The method employscrystal anisotropic etching.

FIGS. 1A to 1D show a semiconductor microstructure 104 formed by themethod of this example and an etching mask 109 used for forming thesemiconductor microstructure. FIG. 1A is a plan view observed from abovethe semiconductor microstructure 104. FIG. 1B is a perspective viewobserved at an angle of about 30° from the position of FIG. 1A. FIGS. 1Cand 1D are sectional views taken along line 1C--1C and line 1D--1D ofFIG. 1A, respectively.

The semiconductor microstructure 104 is an elongate structure as shownin FIGS. 1A and 1B formed in the upper portion of a SIMOX substrate. TheSIMOX substrate is composed of an upper silicon layer (SOI layer), aburied oxide layer, and a lower silicon layer (Si substrate). Thesemiconductor microstructure 104 is formed by etching the upper siliconlayer of the SIMOX substrate. Only a buried oxide layer 105 of the SIMOXsubstrate is shown in FIGS. 1C and 1D as a substrate.

The etching mask 109 used for forming the semiconductor microstructure104 includes a section 103 of an elongate shape as a whole, which ishereinbelow referred to as an "etching control section" of the etchingmask. The etching control section 103 has a first cut 101 and a secondcut 102. The semiconductor microstructure 104 has side edges which arelocated a little inside of ends 111 and 112 of the first and second cuts101 and 102, respectively.

The method for forming the semiconductor microstructure 104 by crystalanisotropic etching of this example will be described with reference toFIGS. 2A to 2C, 3A to 3C, 4A to 4C, and 5A to 5C.

FIG. 2A is a plan view observed from above an etching control section203 of an etching mask 209. FIGS. 2B and 2C are sectional views takenalong line 2B--2B and line 2C--2C of FIG. 2A, respectively.

An n-type SIMOX substrate composed of a lower silicon layer (not shown),the buried oxide layer 105 (thickness: about 400 nm), and an uppersilicon layer 204 (thickness: about 200 nm) is used as the substrate.The SIMOX substrate has a (110) orientation and a <211> direction inorientation flatness. The resistivity of the SIMOX substrate before theformation of the oxide layer is about 10 to 20 Ωcm.

The upper silicon layer 204 is completely oxidized to the bottom thereofin contact with the buried oxide layer 105 by a thermal oxidation filmformation (LOCOS formation) process used in normal Si processing, exceptfor one rectangular region with a size of about 10 μm in width and about21 μm in length. Each of the resultant rectangular regions constitutes adevice region completely isolated from the surroundings.

Thereafter, a silicon oxide film with a thickness of about 75 nm isformed on the device region by pyro-oxidation at about 900° C. for about25 minutes. A resist with a thickness of about 1.5 μm is formed on thesilicon oxide film, and exposed to light and developed with a graystepper, so that the silicon oxide film is patterned by dry etching. Bythis patterning, the etching mask 209 having etching windows (openings)221 and 222 on both sides of the device region is formed from thesilicon oxide film. The etching mask 209 has a stripe-shaped etchingcontrol section 203 between the openings 221 and 222. The etchingcontrol section 203 has cuts 201 and 202 formed on the opposite sides atthe ends thereof.

After the resist is removed, crystal anisotropic etching is performed ata temperature of about 80° C. with a mixture of ethylenediamine,pyrocatechol, and water (mixing ratio: about 6:1:2). The crystalanisotropic etching process will be described in detail.

Since the etching in the <110> direction is about 100 times faster thanthat in the <111> direction, walls which are in contact with the cuts201 and 202 and parallel to the (111) orientation are formed under theetching control section 203 in a short time. Thus, as shown in FIGS. 2Band 2C, uniform (111) walls are formed along the edges of the etchingcontrol section 203.

The etching rate in the <211> direction (about 1500 nm/min.) isextremely (about 200 times) higher than that in the <111> direction(about 7 nm/min.). Accordingly, etching proceeds from an edge 251 of thecut 201 and an edge 252 of the cut 202 toward the center of the etchingcontrol section 203 as shown by the dotted lines in FIG. 2A,substantially simultaneously with the etching in the <110> direction.The etching in the <111> direction (second direction) hardly proceedssince it is extremely slower compared with the etching in the <211>direction (first direction). In other words, in the etching process, theetching rate in the second direction which is vertical to the firstdirection and parallel to the surface of the substrate is substantiallyzero with respect to the etching rate in the first direction.

Stages on the way of the crystal anisotropic etching are shown in FIGS.3A to 3C and 4A to 4C which respectively correspond to FIGS. 2A to 2C.Thus, by this crystal anisotropic etching, the semiconductormicrostructure 104 as shown in FIGS. 5A to 5C is formed.

The width M of the semiconductor microstructure 104 can be easilycontrolled by adjusting the size of the etching control section 203 ofthe etching mask 209. Referring to FIG. 5A, assume that straight linesL1 and L2 which are in level with an end 261 of the cut 201 and an end262 of the cut 202, respectively, and parallel to the <211> directionare drawn. The straight lines L1 and L2 substantially overlap both sideedges of the semiconductor microstructure 104. A distance M between thestraight lines L1 and L2 (i.e., the shortest distance between theopenings 221 and 222 in the <111> direction) determines a width m of thesemiconductor microstructure 104. In this example, the distance M wasset at about 0.2 μm to obtain the semiconductor microstructure 104 withthe width m of about 0.2 μm. More specifically, the width m of theresultant semiconductor microstructure 104 is a little smaller than 0.2μm since the etching in the <111> direction proceeds a little. However,the error is only about 30 nm which can be neglected. Herein, therefore,the width m of the semiconductor microstructure 104 is consideredsubstantially equal to the distance M. The etching amount in the depthdirection can also be easily controlled by use of the SIMOX substrateincluding the buried oxide layer 105. Thus,the semiconductormicrostructure 104 with a height of about 165 nm can be obtained.

Etching masks having etching control sections as shown in FIGS. 6A to 6Dand FIG. 7 may be used, instead of the etching mask having the etchingcontrol section 203 shown in FIG. 5A. A distance M shown in thesefigures corresponds to the distance M between the straight lines L1 andL2 in FIG. 5A. By adjusting the distance M, the width of the resultantsemiconductor microstructure can be easily controlled. FIG. 7 shows atypical size of the etching control section of the etching mask used forthe crystal anisotropic etching.

Thus, according to the present invention, a very narrow semiconductormicrostructure can be formed by only controlling the shape of theetching mask without being affected by the etching rate.

The (111) walls can be formed in a short time by fast etching in the<211> direction while the etching in the <111> direction is slow.Accordingly, a large temporal margin can be secured after thesemiconductor microstructure is completed. That is, when the etchingprocess cannot be terminated immediately after the completion of thesemiconductor microstructure, trouble such as cutting of the completedsemiconductor microstructure and failure in obtaining a desired widthdue to continuing etching in the <111> direction can be prevented.

Even if an angular error arises in the alignment of the etching maskwith the orientation of the substrate, the semiconductor microstructure104 can be formed without being cut halfway, though it is inclined withrespect to the etching control section 103 as shown in FIG. 8. This isbecause, since the cuts are formed on the opposite sides of the etchingcontrol section 103, etching proceeds from these cuts in the <211>direction of the substrate.

In this example, SIMOX was used as the substrate. Alternatively, an SOI(silicon on insulator) substrate, an Si substrate, or a GaAs or InPsubstrate with a silicon layer formed thereon may be used.

(EXAMPLE 2)

Referring to FIGS. 9A to 9D, a method for forming a semiconductormicrostructure of Example 2 will be described. In this method, etchingstop layers are formed to control the length of the semiconductormicrostructure, in addition to the method described in Example 1. FIGS.9A to 9D are plan views observed from above a SIMOX substrate. Since themethod of this example is basically the same as that described inExample 1, only points different from Example 1 will be described.

Referring to FIG. 9A, silicon oxide films 905 and 906 are formed by athermal oxidation film formation process used in normal Si formation toisolate the region of the SIMOX substrate where the semiconductormicrostructure 104 (see FIG. 9D) is to be formed. A distance n betweenthe silicon oxide films 905 and 906 in the <211> direction determinesthe length of the semiconductor microstructure 104 to be formed. In thisexample, the distance n was set at about 5 μm, for example.

The formation of an etching mask having an etching control section 903shown in FIG. 9A and the crystal anisotropic etching process are thesame as those described in Example 1. The crystal anisotropic etchingproceeds as shown in FIGS. 9B and 9C and is finally terminated by thesilicon oxide films 905 and 906 as shown in FIG. 9D. As a result, thesemiconductor microstructure 104 with a desired length (about 5 μm)corresponding to the distance n is formed.

Thus, in this example, by forming the silicon oxide films 905 and 906,the length of the semiconductor microstructure 104 can be controlledwith high precision, and the device region can be isolated from adjacentdevice regions.

A silicon oxide film 1005 as shown in FIGS. 10A to 10E may be formed tocompletely surround the region where the semiconductor microstructure104 is to be formed, instead of the silicon oxide films 905 and 906.FIG. 10E is a sectional view taken along line 10E--10E of FIG. 10A. Thesilicon oxide film 1005 can be formed in the following manner, forexample. A silicon layer (SOI layer) formed on a silicon oxide layer orthe like is patterned by dry-etching to form a silicon island. Then, asilicon oxide film is formed on the patterned silicon layer by thermaloxidation, and selectively removed by etching using a mask, to form thesilicon oxide film 1005 as shown in FIG. 10E. The silicon oxide film1005 may also be formed by a LOCOS formation method.

By using the silicon oxide film 1005 with the above structure, thedevice region can be completely isolated from adjacent device regions,thereby preventing current leakage. The crystal anisotropic etchingprocess in this case is the same as that shown in FIGS. 9A to 9D.

FIG. 14 is a photograph of the semiconductor microstructure taken by SEM(scanning electron microscopy) after the crystal anisotropic etchingaccording to the methods of Examples 1 and 2.

(EXAMPLE 3)

Referring to FIGS. 11A to 11C, a method for fabricating a semiconductordevice of Example 3 will be described. More specifically, a method forfabricating a resonance tunneling device utilizing the resonance tunneleffect will be described.

FIG. 11A is a perspective view of a semiconductor device, FIG. 11B is asectional view taken along line 11B--11B of FIG. 11A, and FIG. 11C is aplan view of the semiconductor device observed from above. As shown inFIGS. 11A and 11B, the semiconductor device is formed on a (110) SOIsubstrate 300 including a silicon substrate 303 and a buried siliconoxide layer 302 formed on the silicon substrate 303. A silicon thinplate (semiconductor microstructure) 305 is formed on the silicon oxidelayer 302 by the crystal anisotropic etching process described inExamples 1 and 2. An etching mask 304 used for shaping the silicon thinplate 305 is formed on the silicon thin plate 305. Oxide barriers 306are formed on both side walls of the silicon thin plate 305 as shown inFIG. 11B. A source 308 and a drain 309 are formed on the opposite sidesof the silicon thin plate 305 in contact with the oxide barriers 306under the etching mask 304. The silicon thin plate 305 is thus flankedby the silicon oxide barriers 306, forming a double barrier structure.

FIG. 12 shows the electric characteristics of the semiconductor deviceshown in FIGS. 11A to 11C. As is observed from FIG. 12, thesemiconductor device exhibits a so-called negative resistance where acurrent I is affected by the resonance tunnel effect thereby repeatingincrease and decrease by the application of a voltage to the drain 309relative to the source 308.

Hereinbelow, the method for fabricating the resonance tunneling deviceof this example will be described with reference to FIGS. 13A to 13Fwhich are perspective views illustrating the fabrication steps.

As shown in FIG. 13A, a mask 404 made of SiO₂ (thickness: about 75 nm)parallel to the <211> direction is formed on a (110) SOI substrate 400composed of an upper silicon layer 401 (thickness: about 200 nm), aburied silicon oxide layer 402 (thickness: about 400 nm), and a (110) Sisubstrate 403. The mask 404 (etching control section) has a shape andsize as shown in FIG. 7.

As shown in FIG. 13B, crystal anisotropic etching is performed with amixture of ethylenediamine, pyrocatechol, and water (mixing ratio: about6:1:2) at about 80° C. The crystal anisotropic etching proceeds in adirection shown by an arrow in FIG. 13B due to the orientationdependency of the etching rate, and finally, a silicon thin plate 405(width: about 0.2 μm, length: about 5 μm, height: about 165 nm) having(111) side walls is formed under the mask 404 as shown in FIG. 13C.

As shown in FIG. 13D, silicon oxide barriers 406 which are thin enoughto allow electrons to pass therethrough by the tunnel effect (thickness:about 1.5 nm) are formed on the both side walls of the silicon thinplate 405 by thermal oxidation.

As shown in FIG. 13E, polysilicon 410 is deposited on the entire surfaceof the substrate and implanted with impurities. The depositedpolysilicon 410 is dry-etched using the mask 404, so that a source 408and a drain 409 as electrodes are formed under the mask 404 in aself-alignment fashion as shown in FIG. 13F.

As described above, the mask 404 is used as the mask for the etching ofthe polysilicon 410 in the step of forming the source 408 and the drain409, as well as for etching the upper silicon layer in the step offorming the silicon thin plate 405. Accordingly, in the fabrication of adevice with the configuration as shown in FIG. 13F, only one step isrequired for forming the mask required. In other words, no additionalmask is required at the formation of the source and the drain.

In this example, therefore, the fabrication process can be simplifiedsince only one step is required for forming the mask required. As in thepreceding examples, the silicon thin plate 405 (semiconductormicrostructure) can be formed with good controllability depending on theshape of the mask 404.

In this example, by using the SOI substrate having the buried siliconoxide layer 402, current leakage through the substrate can be prevented.The height of the silicon thin plate 405 to be formed later can becontrolled by adjusting the thickness of the upper silicon layer 401 ofthe SOI substrate before the formation of the etching mask. Thus, thecurrent can be set at an appropriate value by changing the area wherethe current flows, so as to adjust the capacitance between the sourceand drain electrodes and thereby improve the operation speed of thedevice. The same results can also be obtained by using a SIMOXsubstrate.

Thus, according to the present invention, the following effects can beobtained.

The width of the semiconductor microstructure to be formed can be easilycontrolled by adjusting the depth of the cuts of the etching controlsection of the etching mask. A very narrow semiconductor microstructurecan be formed only by the control of the shape of the etching maskwithout being affected by the rate of the crystal anisotropic etching.

Since the cuts are formed on the opposite sides of the etching controlsection, the semiconductor microstructure can be formed without beingcut halfway even if an angular error arises in the alignment of theetching mask with the orientation of the substrate.

The (111) walls of the semiconductor microstructure can be formed in ashort time by use of faster etching in the <211> direction. A largetemporal margin can be secured after the semiconductor microstructure iscompleted, since the etching rate in the <111> direction is slow.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A method for forming a semiconductormicrostructure, comprising the steps of:forming a mask pattern having afirst opening and a second opening on a substrate having a semiconductorlayer as an upper portion thereof; and selectively etching thesemiconductor layer using the mask pattern to form a semiconductormicrostructure extending in a first direction parallel to a surface ofthe substrate, wherein, in the step of selectively etching thesemiconductor layer, an etching rate in a second direction vertical tothe first direction and parallel to the surface of the substrate issubstantially zero with respect to an etching rate in the firstdirection, and a width of the semiconductor microstructure issubstantially equal to a shortest distance between the first opening andthe second opening in the second direction.
 2. A method for forming asemiconductor microstructure according to claim 1, wherein theorientation of the substrate is (110), the first direction is <211>, andthe second direction is <111>.
 3. A method for forming a semiconductormicrostructure according to claim 1, wherein the substrate is an SOIsubstrate.
 4. A method for forming a semiconductor microstructureaccording to claim 1, wherein the substrate is a SIMOX substrate.
 5. Amethod for forming a semiconductor microstructure according to claim 1,wherein the mask pattern has a stripe-shaped portion between the firstopening and the second opening, and a length direction of thestripe-shaped portion does not correspond to the first direction.
 6. Amethod for forming a semiconductor microstructure according to claim 1,further comprising the step of forming a first oxide film and a secondoxide film formed in parallel to each other in the first direction tosandwich the semiconductor layer, wherein, in the step of selectivelyetching the semiconductor layer, etching in the first direction isterminated by the first oxide film and the second oxide film.
 7. Amethod for forming a semiconductor microstructure according to claim 1,wherein the semiconductor layer is made of Si.
 8. A method for forming asemiconductor microstructure according to claim 1, wherein an oxide filmis formed under the semiconductor layer of the substrate.
 9. A methodfor forming a semiconductor microstructure according to claim 1, whereinthe step of selectively etching the semiconductor layer includes crystalanisotropic etching.
 10. A method for fabricating a resonance tunnelingdevice including a silicon thin plate having side walls and a width thinenough to serve as a quantum well, a pair of tunnel barriers formed onthe side walls of the silicon thin plate, and a pair of electrodesformed to sandwich the pair of tunnel barriers, wherein the silicon thinplate is formed in the steps of:forming an oxide film having a firstopening and a second opening on a substrate having a silicon layer as anupper portion thereof; and selectively etching the silicon layer usingthe oxide film as a mask to form the silicon thin plate extending in afirst direction parallel to a surface of the substrate, wherein, in thestep of selectively etching the silicon layer, an etching rate in asecond direction vertical to the first direction and parallel to thesurface of the substrate is substantially zero with respect to anetching rate in the first direction, and a width of the silicon thinplate is substantially equal to a shortest distance between the firstopening and the second opening in the second direction.
 11. A method forfabricating a resonance tunneling device according to claim 10, whereinthe orientation of the substrate is (110), the first direction is <211>,and the second direction is <111>.
 12. A method for fabricating aresonance tunneling device according to claim 10, wherein the substrateis an SOI substrate.
 13. A method for fabricating a resonance tunnelingdevice according to claim 10, wherein the substrate is a SIMOXsubstrate.
 14. A method for fabricating a resonance tunneling deviceaccording to claim 10, wherein the mask pattern has a stripe-shapedportion between the first opening and the second opening, and a lengthdirection of the stripe-shaped portion does not correspond to the firstdirection.
 15. A method for fabricating a resonance tunneling deviceaccording to claim 10, further comprising the step of forming a firstoxide film and a second oxide film formed in parallel to each other inthe first direction to sandwich the semiconductor layer, wherein, in thestep of selectively etching the silicon layer, etching in the firstdirection is terminated by the first oxide film and the second oxidefilm.
 16. A method for fabricating a resonance tunneling deviceaccording to claim 10, wherein the step of selectively etching thesilicon layer includes crystal anisotropic etching.
 17. A method forfabricating a resonance tunneling device according to claim 10, whereinthe pair of electrodes are formed by dry etching using the oxide film asa mask in a self-alignment fashion.